Exhaust emission control device and method of controlling exhaust emission

ABSTRACT

A first heat-resistant filter medium is disposed in an exhaust pathway of an internal combustion engine. The first heat-resistant filter medium traps hydrocarbon compounds and carbon-containing particulates included in a flow of exhaust gas in a dispersive manner to bring the respective particulates and hydrocarbon compounds in contact with oxygen included in the exhaust gas. The trapped hydrocarbon compounds and the trapped carbon-containing particulates are subjected to combustion with the exhaust gas having a filter inflow temperature lower than a combustible temperature of the carbon-containing particulates. A second heat-resistant filter medium is further disposed downstream of the first heat-resistant filter medium to trap the remaining carbon-containing particulates, which have not been trapped by the first heat-resistant filter medium but have passed through the first heat-resistant filter medium. This arrangement desirably enhances the reduction rate of the particulates. Since most of the particulates are trapped by the first filter medium, the second filter medium can keep the high reduction rate of the particulates over a long time period.

TECHNICAL FIELD

[0001] The present invention relates to a technique of regulating andreducing particulates included in exhaust gases from an internalcombustion engine.

BACKGROUND ART

[0002] The exhaust gas from internal combustion engines, especiallyDiesel engines, includes carbon-containing particulates like black smoke(soot), and there is a high demand of reducing the total emission of thecarbon-containing particulates, in order to prevent further airpollution. There is a similar demand in direct injection gasolineengines where gasoline is directly injected into a combustion chamber,since the carbon-containing particulates may be discharged with theexhaust gas under some driving conditions.

[0003] One proposed method of remarkably reducing the carbon-containingparticulates in the emission from an internal combustion engine hasdisposes a heat-resistant filter in an exhaust conduit of the internalcombustion engine and uses the filter to trap the carbon-containingparticulates included in the exhaust gas. This method significantlyreduces the total quantity of the carbon-containing particulatesreleased to the air, while requiring treatment of the trappedcarbon-containing particulates to avoid potential troubles like theclogging of the filter and the lowered performance of the engine.

[0004] Several methods have been proposed to treat the trappedcarbon-containing particulates. One proposed technique makes a noblemetal catalyst, such as platinum, carried on the filter and utilizes thecatalytic action of the noble metal for combustion (see JAPANESE PATENTPUBLICATION GAZETTE No. 7-106290). Another proposed techniqueintentionally raises the temperature of the exhaust gas for combustionof the trapped carbon-containing particulates on the filter (see PATENTAPPLICATION No. 2000-161044). Combustion of the carbon-containingparticulates by application of any of these techniques ensures treatmentof the particulates prior to clogging of the filter. The filter havingthe higher trapping rate lowers the total quantity of thecarbon-containing particulates released to the air.

[0005] The catalyst naturally deteriorates its performance in use. Thecatalyst used for a long time period can not completely treat thetrapped carbon-containing particulates. This eventually leads toclogging of the filter. The technique of intentionally raising thetemperature of the exhaust gas takes out the chemical energy of the fuelnot in the form of the output of the engine but in the form of heat.This technique thus disadvantageously lowers the output of the engine orthe fuel consumption efficiency.

[0006] By taking into account these problems, the inventors of thepresent invention have completed a technique of readily treating thetrapped carbon-containing particulates and have filed for a patentapplication (PATENT APPLICATION No. 2000-300167). The techniquedisclosed in the application uses a heat-resistant filter to trap thecarbon-containing particulates and the hydrocarbon compounds included inthe flow of exhaust gas in a dispersive manner to bring the respectiveparticulates and hydrocarbon compounds in contact with oxygen includedin the exhaust gas. The dispersive trapping causes the hydrocarboncompounds to be gradually oxidized with oxygen in the exhaust gas, evenwhen a filter inflow temperature of the exhaust gas is lower than acombustible temperature of the carbon-containing particulates. Highlyactive intermediate products and reaction heat produced through theoxidation are accumulated and eventually cause combustion of thecarbon-containing particulates. Application of this technique enablesthe carbon-containing particulates to be effectively treated by simplymaking the carbon-containing particulates and the hydrocarbon compoundsin the exhaust gas trapped on the filter in the dispersive manner. Thisis free from the problems like the clogging of the filter due todeterioration of the catalyst and the lowered performance of the engine.

[0007] It is, however, practically not easy for the filter to trap allthe carbon-containing particulates and the hydrocarbon compoundsincluded in the flow of exhaust gas in the dispersive manner to bringthe respective particulates and hydrocarbon compounds in contact withoxygen included in the exhaust gas. It is thus highly probable that atrace amount of the carbon-containing particulates passes through thefilter and is released to the air. The higher trapping rate of thefilter to prevent the release makes it difficult to trap thecarbon-containing particulates and the hydrocarbon compounds in adispersive manner. This leads to failure of combustion of the trappedcarbon-containing particulates with the exhaust gas of relatively lowtemperature.

[0008] The present invention has been completed to solve the drawbacksof the prior art techniques discussed above and to improve the techniqueof the pending patent application mentioned above. The object of thepresent invention is thus to stably control carbon-containingparticulates included in the flow of exhaust gas from an internalcombustion engine over a long time period without deteriorating theperformances of the internal combustion engine and to reduce the totalquantity of particulates released to the air.

DISCLOSURE OF THE INVENTION

[0009] At least part of the above and the other related objects isattained by a first emission control device that reducescarbon-containing particulates included in a flow of exhaust gas from aninternal combustion engine. The first emission control device includes:a first heat-resistant filter medium that traps hydrocarbon compoundsand the carbon-containing particulates included in the flow of exhaustgas in a dispersive manner to bring the respective particulates andhydrocarbon compounds in contact with oxygen included in the exhaustgas, and thereby makes the trapped hydrocarbon compounds and the trappedcarbon-containing particulates subjected to combustion with the exhaustgas having a filter inflow temperature lower than a combustibletemperature of the carbon-containing particulates; and a secondheat-resistant filter medium that traps the remaining carbon-containingparticulates, which have not been trapped by the first heat-resistantfilter medium but have passed through the first heat-resistant filtermedium.

[0010] There is an emission control method corresponding to the aboveemission control device.

[0011] The present invention is accordingly directed to a first emissioncontrol method that reduces carbon-containing particulates included in aflow of exhaust gas from an internal combustion engine. The firstemission control method includes the steps of: using a firstheat-resistant filter medium to trap hydrocarbon compounds and thecarbon-containing particulates included in the flow of exhaust gas in adispersive manner to bring the respective particulates and hydrocarboncompounds in contact with oxygen included in the exhaust gas; making thetrapped hydrocarbon compounds and the trapped carbon-containingparticulates subjected to combustion with the exhaust gas having aninflow temperature into the first heat-resistant filter medium lowerthan a combustible temperature of the carbon-containing particulates;and using a second heat-resistant filter medium to trap the remainingcarbon-containing particulates, which have not been trapped by the firstheat-resistant filter medium but have passed through the firstheat-resistant filter medium.

[0012] In the first emission control device and the corresponding firstemission control method, the first heat-resistant filter medium disposedupstream traps the carbon-containing particulates included in the flowof exhaust gas, and the second heat-resistant filter medium disposeddownstream of the first heat-resistant filter medium traps the remainingcarbon-containing particulates, which have not been trapped by the firstheat-resistant filter medium but have passed through the firstheat-resistant filter medium.

[0013] The arrangement of the present invention uses the secondheat-resistant filter medium to trap and treat the remainingcarbon-containing particulates passing through the first heat-resistantfilter medium, thus significantly reducing the total quantity of thecarbon-containing particulates released to the air.

[0014] In accordance with one preferable application of the emissioncontrol device, the second heat-resistant filter medium is capable oftrapping the remaining carbon-containing particulates, which are smallerin size than the carbon-containing particulates collectable by the firstheat-resistant filter medium.

[0015] This arrangement advantageously enables the carbon-containingparticulates, which have not been trapped by the first heat-resistantfilter medium but have passed through the first heat-resistant filtermedium, to be effectively trapped by the second heat-resistant filtermedium. In general, the filter material that is capable of trapping thefiner carbon-containing particulates has the higher chance of clogging.In this arrangement, however, since most of the carbon-containingparticulates in the exhaust gas are trapped by the first heat-resistantfilter medium, application of the filter material that is capable oftrapping the finer particulates to the second heat-resistant filtermedium effectively reduces the total quantity of the particulatesreleased to the air without the fear of clogging.

[0016] When the internal combustion engine is provided with a pluralityof combustion chambers, an exhaust manifold that unites flows of exhaustgas from the plurality of combustion chambers to at least one jointflow; and an exhaust pipe that leads the joint flow of exhaust gasunited by the exhaust manifold to the air, in one preferable structureof the emission control device, the first heat-resistant filter mediumis disposed in the exhaust manifold, and the second heat-resistantfilter medium is disposed in the exhaust pipe.

[0017] In this layout, the first heat-resistant filter medium is closedto the internal combustion engine, so that high-temperature exhaust gasis flown into the first heat-resistant filter medium. This facilitatescombustion of the trapped carbon-containing particulates. Thearrangement of disposing the second heat-resistant filter medium in theexhaust pipe after the exhaust manifold that unites the flows of exhaustgas desirably facilitates replacement of the second heat-resistantfilter medium.

[0018] In the emission control device of the above layout, the firstheat-resistant filter medium may be disposed at a specific positionwhere the flows of exhaust gas from the plurality of combustion chambersare united to the at least one joint flow, in the exhaust manifold. Inthis arrangement, the first heat-resistant filter medium can be locatedin a relatively wide space. The wider space heightens the degree offreedom in shape of the filter medium and allows the filter medium tohave more adequate shape and size. The specific position in the exhaustmanifold, where the flows of exhaust gas from the respective combustionchambers are united to the at least one joint flow, are not apart fromthe combustion chambers. The exhaust gas of still high temperature isaccordingly flown into the first heat-resistant filter medium disposedat the specific position. This factor, in combination of the adequateshape and size of the filter medium, ensures effective combustion of thetrapped carbon-containing particulates.

[0019] In the emission control device, a filter material that does nottrap most of metal sulfate particulates but allows passage of the metalsulfate particulates therethrough may be applied for the firstheat-resistant filter medium. Here the metal sulfate particulates areproduced from metal components added to lubricating oil of the internalcombustion engine and sulfur in a fuel of the internal combustion engineand are suspended in the flow of exhaust gas.

[0020] The metal sulfates have extremely high thermal stability. If thefirst heat-resistant filter medium traps the metal sulfates in theexhaust gas, there is difficulty in treating the trapped particulates.This may cause clogging of the first heat-resistant filter medium.Application of the filter material that does not trap most of the metalsulfates but allows passage of the metal sulfates to the firstheat-resistant filter medium desirably prevents the first heat-resistantfilter medium from being clogged.

[0021] In accordance with one preferable embodiment, the emissioncontrol device further has a vane that is located on a pathway of theflow of exhaust gas from the internal combustion engine, is driven bythe flow of exhaust gas, and breaks down the particulates included inthe flow of exhaust gas. The first heat-resistant filter medium isdisposed upstream of the vane, and the second heat-resistant filtermedium is disposed downstream of the vane.

[0022] In this embodiment, the high-temperature exhaust gas is flowninto the first heat-resistant filter medium to facilitate combustion ofthe trapped carbon-containing particulates. The particulates passingthrough the first heat-resistant filter medium are crushed by the vaneand are thus more readily allowed to pass through the secondheat-resistant filter medium. This arrangement desirably prevents thesecond heat-resistant filter medium from being clogged with the hardlycombustible particulates like the metal sulfates.

[0023] When the internal combustion engine is provided with asupercharger that utilizes fluidization energy of the exhaust gas tosupercharge intake air of the internal combustion engine, the vane ofthe emission control device may be a turbine of the superchargeractuated by the flow of exhaust gas.

[0024] The turbine of the supercharger rotates at a high speed andeffectively crushes down the particulates included in the exhaust gas.This desirably prevents the second heat-resistant filter medium frombeing clogged.

[0025] In the emission control device applied to the internal combustionengine with the supercharger, a control catalyst may be disposed in backwash of the second heat-resistant filter medium to reduce air pollutantsin the exhaust gas passing through the second heat-resistant filtermedium. The control catalyst functions to treat gaseous air pollutantsin the exhaust gas, such as carbon monoxide and SOF (Soluble OrganicFraction), prior to release of the exhaust gas to the air.

[0026] In the emission control device, a filter material with an activeoxygen release agent carried thereon to take in and hold oxygen in thepresence of excess oxygen in its atmosphere and release the oxygen heldtherein as active oxygen with a decrease in concentration of oxygen inthe atmosphere may be applied for the second heat-resistant filtermedium.

[0027] The active oxygen is highly reactive and thus quickly oxidizesthe carbon-containing particulates trapped on the second heat-resistantfilter medium to convert the carbon-containing particulates intoharmless substances like carbon dioxide and water. The active oxygenrelease agent carried on the second heat-resistant filter mediumreleases active oxygen with a variation in concentration of oxygen inthe exhaust gas, which is accompanied by a variation in drivingconditions of the internal combustion engine, and thereby effectivelytreats the trapped carbon-containing particulates.

[0028] In the emission control device of the present invention, most ofthe carbon-containing particulates included in the exhaust gas aretrapped by the first heat-resistant filter medium, so that there ispractically no chance that a large quantity of the carbon-containingparticulates are flown into the second heat-resistant filter medium. Itis thus unlikely that the surface of the second heat-resistant filtermedium is covered with the large amount of inflow carbon-containingparticulates and can not take in excess oxygen or release active oxygen.

[0029] In the emission control device of the above embodiment, a filtermaterial with a noble metal catalyst belonging to a platinum groupcarried thereon in addition to the active oxygen release agent may beapplied for the second heat-resistant filter medium.

[0030] As is known in the art, the noble metal belonging to the platinumgroup has moderate oxidation activity when being used as the catalyst.The reaction of taking in excess oxygen in the exhaust gas and releasingthe intake excess oxygen as active oxygen with a decrease of the oxygenconcentration is a redox reaction as described later. Carriage of thenoble metal, which belongs to the platinum group and has moderateoxidation activity, in addition to the active oxygen release agentaccelerates the reaction of taking in excess oxygen and releasing activeoxygen, thereby effectively treating the carbon-containing particulatestrapped on the second heat-resistant filter medium.

[0031] In order to solve at least part of the problems of the prior artdiscussed above, the present invention is also directed to a secondemission control device that reduces carbon-containing particulates,which are included in a flow of exhaust gas with a variation in flowrate emitted from an internal combustion engine, using a filter materialhaving a large number of pores tangled in a three-dimensional manner.The second emission control device includes: a first heat-resistantfilter medium that is composed of the filter material, makes the exhaustgas flown into the pores, which are greater in size than thecarbon-containing particulates, and causes the carbon-containingparticulates to collide with and adhere to regions defining the pores ofthe filter material, thereby trapping the carbon-containingparticulates; a second heat-resistant filter medium that filters theflow of exhaust gas passing through the first heat-resistant filtermedium to trap the remaining carbon-containing particulates included inthe flow of exhaust gas; and a flow rate variation mitigation modulethat mitigates the variation in flow rate of the exhaust gas flown intothe second heat-resistant filter medium.

[0032] There is an emission control method corresponding to the aboveemission control device. The present invention is accordingly directedto a second emission control method that reduces carbon-containingparticulates, which are included in a flow of exhaust gas with avariation in flow rate emitted from an internal combustion engine, usinga filter material having a large number of pores tangled in athree-dimensional manner. The second emission control method includesthe steps of: making the exhaust gas flown into the pores, which aregreater in size than the carbon-containing particulates, and causing thecarbon-containing particulates to collide with and adhere to regionsdefining the pores of the filter material, thereby trapping thecarbon-containing particulates; mitigating the variation in flow rate ofthe exhaust gas; and filtering the flow of exhaust gas with themitigated variation in flow rate, thereby trapping the remainingcarbon-containing particulates included in the flow of exhaust gas.

[0033] In the second emission control device and the correspondingsecond emission control method of the present invention, the exhaust gasincluding the carbon-containing particulates with a variation in flowrate is flown into the first heat-resistant filter medium. The firstheat-resistant filter medium has pores, which are greater in size thanthe carbon-containing particulates. As the exhaust gas is flown intosuch pores, the carbon-containing particulates collide with and adhereto the regions defining the pores of the filter material. Namely thisprocess dynamically traps the carbon-containing particulates. Theexhaust gas passing through the first heat-resistant filter medium isthen filtered by the second heat-resistant filter medium. This processstatically traps the remaining carbon-containing particulates in theexhaust gas. In the static trapping process, the exhaust gas with themitigated variation in flow rate is flown into the second heat-resistantfilter medium. The terms ‘dynamically trap’ and ‘statically trap’ willbe explained later.

[0034] This arrangement enables the carbon-containing particulatesincluded in the exhaust gas to be efficiently treated and reducedwithout clogging the filter medium, because of the reasons discussedbelow.

[0035] The first heat-resistant filter medium causes thecarbon-containing particulates to collide with and adhere to the regionsdefining the pores of the filter material and thus dynamically traps thecarbon-containing particulates. This mechanism is described withreference to FIG. 17. FIG. 17(a) conceptually shows the flow of exhaustgas through the pores formed inside the first heat-resistant filtermedium. The hatched portions schematically represent members definingthe pores. The flow of exhaust gas passes through the pores defined bythese members. The arrows schematically represent the flows of exhaustgas passing through the pores. The pores formed inside theheat-resistant filter medium are tangled in a three-dimensional manner.The exhaust gas passes through the pores, while often changing its flowdirection as illustrated. In the course of changing the flow directionof the exhaust gas, small particulate readily changes its direction andgoes on the flow of exhaust gas to pass through the pores. Largeparticulate, however, can not readily change its direction but collideswith the inner face of the pores.

[0036]FIG. 17(b) is a conceptual view showing this aspect in detail. Thearrow of the solid line represents the flow of exhaust gas, and thearrow of the broken line represents the flow of particulate in theexhaust gas. As the flow of exhaust gas changes its direction, a smallparticulate Ps goes on the flow of exhaust gas with the changingdirection. A large particulate PL, on the other hand, does not changeits direction with the change of the flow direction of the exhaust gasbut collides with the inner face of a pore. The carbon-containingparticulates in the exhaust gas include a wet fraction fromnon-combusted fuel and engine oil. The particulate colliding with theinner face of the pore adheres to the inner face of the pore and isthereby trapped therein by the function of the wet fraction.

[0037] Changing the flow direction of the particulate in the exhaust gaswith the changed flow direction of the exhaust gas is attributed to theviscosity of the fluid (that is, the exhaust gas). Immediately after thechange of the flow direction of the exhaust gas, the particulate movesin the former direction according to the law of inertia. The particulatethen comes across the flow of exhaust gas and meets resistance of theflow of exhaust gas. More specifically, since the particulate and theexhaust gas surrounding the particulate have different flow directions,a significant velocity gradient occurs on the surface of theparticulate. A force of the velocity gradient multiplied with aviscosity acts on the particulate. The particulate in the exhaust gaschanges its flow direction by this viscosity-based force. Namely thephenomenon of changing the flow direction of the particulate with thechanged flow direction of the exhaust gas is attributed to theviscosity-based force of the fluid acting on the particulate. By takinginto account such attribution, the degree of easiness of changing theflow direction of the particulate with the changed flow direction of theexhaust gas is expressible with a Reynolds number Re. The Reynoldsnumber Re is a dimensionless number expressed by the equation of:

Re=Ud/v

[0038] where U, d, and v respectively denote the flow rate, the size ofthe particulate, and the dynamic viscosity of the exhaust gas. TheReynolds number physically represents the effect of the viscosity of thefluid on the state of the flow. The smaller Reynolds number Re resultsin the greater effect of the viscosity on the flow. The larger Reynoldsnumber Re, on the contrary, results in the greater effect of inertia onthe flow.

[0039] As clearly understood from the above equation, the smallerparticle diameter of the carbon-containing particulates included in theexhaust gas leads to the smaller Reynolds number Re and the greatereffect of the viscosity. The particulate then readily changes its flowdirection and goes on the flow of exhaust gas to pass through the pores.The greater particle diameter of the carbon-containing particulates, onthe other hand, leads to the larger Reynolds number Re. This relativelyreduces the effect of the viscosity but enhances the effect of inertia.As the exhaust gas changes its flow direction, the particulate does notreadily changes its flow direction but collides with the inner face ofthe pore to be trapped therein. In the specification hereof, the term‘dynamically trap’ represents trapping the particulate through collisionand adhesion according to the mechanism discussed above. The firstheat-resistant filter medium dynamically trap the carbon-containingparticulates included in the exhaust gas. This process mainly trapslarge particulates.

[0040] The subsequent second heat-resistant filter medium filters theexhaust gas, which has passed through the first heat-resistant filtermedium, so as to statically trap the remaining smaller carbon-containingparticulates included in the exhaust gas. The second heat-resistantfilter medium having a large number of small pores or narrow gaps isused to filter the exhaust gas including the carbon-containingparticulates and thereby trap the particulates that can not pass throughthe pores or gaps. In the specification hereof, the term ‘staticallytrap’ represents the state of gently filtering off and trapping theparticulates in the exhaust gas without causing collision or adhesion.The ‘dynamically trapping’ process and the ‘statically trapping’ processadopt significantly different mechanisms for trapping. The process of‘dynamically trapping’ traps the particulates by making the exhaust gasflown into the pores that are greater in size than the particulates. Theprocess of ‘statically trapping’, on the other hand, traps theparticulates by making the exhaust gas flown into the pores or gaps thatare practically equivalent or smaller in size to or than theparticulates.

[0041] In the case where the filter material having small pores ornarrow gaps is applied to ‘statically trap’ the particles in the exhaustgas, the small pores or the narrow gaps are soon clogged with largeparticulates. The arrangement of ‘dynamically trapping’ the largeparticulates in the exhaust gas before ‘statically trapping’ theremaining particulates in the exhaust gas desirably prevents the filtermaterial, which statically filters the exhaust gas, from being clogged.

[0042] In the second emission control device and the second emissioncontrol method of the present invention, while the exhaust gas with thevariation in flow rate is flown into the heat-resistant filter medium,the exhaust gas with the mitigated variation in flow rate is flown intothe second heat-resistant filter medium. This arrangement furtherprevents clogging of the filter medium and thus more efficiently trapsthe particulates. When the exhaust gas flown into the firstheat-resistant filter medium has the variation in flow rate, thecarbon-containing particulates included in the exhaust gas are flowninto the pores at a higher speed corresponding to the variation in flowrate. This results in collision and adhesion of even smallerparticulates. The increase in rate of the flow into the pores raises theReynolds number Re described above to have the greater effects ofinertia on even the small particulates. The particulates are thus apt tocollide with the inner face of the pores without changing the directionof the flow.

[0043] The first heat-resistant filter medium effectively traps thelarge carbon-containing particulates according to the mechanismdiscussed above. The small carbon-containing particulates, however, goon the flow of exhaust gas and pass through the first heat-resistantfilter medium. The flow of exhaust gas passing through the firstheat-resistant filter medium is then led into the second heat-resistantfilter medium. Since the flow of exhaust gas has the mitigated variationin flow rate, the small particulates do not move around the surface ofthe second heat-resistant filter medium due to the varying flow rate ofthe exhaust gas. The small carbon-containing particulates in the exhaustgas can thus be quickly trapped by the second heat-resistant filtermedium.

[0044] The carbon-containing particulates in the exhaust gas include thewet fraction from the non-combusted fuel and engine oil as mentionedabove. The small particulate moving around the surface of the secondheat-resistant filter medium is combined with surrounding particulatesand grown to a greater size. The grown particulates are likely to clogthe second heat-resistant filter medium. When the flow of exhaust gaswith the mitigated variation in flow rate is led into the secondheat-resistant filter medium, the second heat-resistant filter medium isnot clogged with the grown carbon-containing particulates butefficiently traps the particulates.

[0045] As described above, the second emission control device and thecorresponding second emission control method of the present inventioncause the exhaust gas with the variation in flow rate to be flown intothe first heat-resistant filter medium. The first heat-resistant filtermedium can thus dynamically trap even relatively small carbon-containingparticulates. This causes only smaller carbon-containing particulates tobe flown into the second heat-resistant filter medium and thuseffectively prevents the second heat-resistant filter medium from beingclogged. The exhaust gas flown into the second heat-resistant filtermedium has the mitigated variation in flow rate. The mitigated variationin flow rate enables the second heat-resistant filter medium to quicklytrap the small carbon-containing particulates and prevents aggregationof particulates, which may cause clogging of the filter medium.

[0046] The exhaust gas includes metal sulfate particulates, which areproduced from metal components added to lubricating oil of the internalcombustion engine and sulfur in the fuel of the internal combustionengine, in addition to the carbon-containing particulates as mentionedpreviously. The metal sulfate particulates are not large enough to bedynamically trapped by the first heat-resistant filter medium. The firstheat-resistant filter medium is thus not clogged with the thermallystable metal sulfate particulates. The particulates passing through thefirst heat-resistant filter medium may be trapped by the secondheat-resistant filter medium. The second heat-resistant filter medium isdisposed at a location allowing easy access for maintenance, comparedwith the first heat-resistant filter medium. Even if the secondheat-resistant filter medium is clogged, the second heat-resistantfilter medium is thus readily accessible for maintenance.

[0047] In accordance with one preferable application of the secondemission control device of the present invention, the firstheat-resistant filter medium is composed of a filter material that trapsthe hydrocarbon compounds and the carbon-containing particulatesincluded in the flow of exhaust gas in a dispersive manner to bring therespective particulates and hydrocarbon compounds in contact with oxygenincluded in the exhaust gas and thereby makes the trapped hydrocarboncompounds and the trapped carbon-containing particulates subjected tocombustion with the exhaust gas having a filter inflow temperature lowerthan a combustible temperature of the carbon-containing particulates.The second heat-resistant filter medium is composed of a filter materialwith an active oxygen release agent carried thereon to take in and holdoxygen in the presence of excess oxygen in its atmosphere and releasethe oxygen held therein as active oxygen with a decrease inconcentration of oxygen in the atmosphere.

[0048] The first heat-resistant filter medium that is capable oftrapping relatively large carbon-containing particulates traps thehydrocarbon compounds and the carbon-containing particulates included inthe flow of exhaust gas in a dispersive manner to bring the respectiveparticulates and hydrocarbon compounds in contact with oxygen includedin the exhaust gas. This ensures combustion of the trappedcarbon-containing particulates. Since the relatively largecarbon-containing particulates have been trapped by the firstheat-resistant filter medium, the second heat-resistant filter mediummainly traps relatively small carbon-containing particulates. Therelatively small carbon-containing particulates trapped on the secondheat-resistant filter medium are quickly treated by active oxygen. Suchquick treatment effectively prevents the second heat-resistant filtermedium from being clogged. Such quickly treatment of the trappedparticulates facilitates further trapping of the particulates and thusenhances the trapping efficiency of the carbon-containing particulate s.

[0049] In one preferable embodiment of the emission control device ofthe above arrangement, a supercharger that is actuated by fluidizationenergy of the exhaust gas and supercharges intake air of the internalcombustion engine is located between the first heat-resistant filtermedium and the second heat-resistant filter medium as the means ofmitigating the variation in flow rate of the exhaust gas flown into thesecond heat-resistant filter medium. The supercharger works as ahydrodynamic flow-restriction element. Passage through the superchargermitigates the variation in flow rate of the exhaust gas. This allows thesecond heat-resistant filter medium to efficiently trap the smallcarbon-containing particulates in the exhaust gas and effectivelyprevents the second heat-resistant filter medium from being clogged,because of the reason explained previously.

[0050] The means of mitigating the variation in flow rate is notrestricted to the supercharger, but may narrow the pathway of theexhaust gas between the first heat-resistant filter medium and thesecond heat-resistant filter medium, may have an orifice, or may have ahydrodynamic volume element. The volume element represents a tank-shapedportion inserted in the middle of the pathway of the exhaust gas. Any ofsuch means provided in the middle of the pathway of the exhaust gasensures mitigation of the variation in flow rate of the exhaust gas. Thesupercharger is, however, preferable since it has an additional functionto enhance the output of the internal combustion engine.

[0051] In one preferable embodiment of the emission control device withthe supercharger located between the first heat-resistant filter mediumand the second heat-resistant filter medium, a flow-restriction elementis disposed in back wash of the second heat-resistant filter medium. Theflow-restriction element has an orifice or otherwise narrows the pathwayof the exhaust gas to intentionally heighten the flow resistance andthereby restrict the flow of the exhaust gas. The flow-restrictionelement disposed in back wash of the second heat-resistant filtermedium, in addition to the supercharger advantageously attains furthermitigation of the variation in flow rate of the exhaust gas flown intothe second heat-resistant filter medium.

[0052] The flow-restriction element disposed in back wash of the secondheat-resistant filter medium may be a control catalyst that reduces airpollutants included in the flow of exhaust gas. Gaseous air pollutantslike carbon monoxide and SOF (soluble organic fraction), in addition tothe carbon-containing particulates are included in the flow of exhaustgas. The control catalyst disposed in back wash of the secondheat-resistant filter medium as the flow-restriction element desirablyreduces such air pollutants. Even if the air pollutants like carbonmonoxide are produced by some reason in the course of combustion of thecarbon-containing particulates trapped by the first heat-resistantfilter medium, the control catalyst advantageously prevents the airpollutants from being released to the air.

BRIEF DESCRIPTION OF THE DRAWINGS

[0053]FIG. 1 illustrates the construction of a Diesel engine with anemission control device of a first embodiment applied thereto;

[0054]FIG. 2 shows the appearance and the structure of a particulatefilter in the embodiment;

[0055]FIG. 3 conceptually shows a process of trapping particulatesincluded in a flow of exhaust gas by means of the particulate filter ofthe embodiment;

[0056]FIG. 4 shows dimensions of non-woven fabrics applicable to theparticulate filter of the embodiment;

[0057]FIG. 5 conceptually shows a process of trapping particulatesincluded in the flow of exhaust gas by means of another particulatefilter in one modified example of the embodiment;

[0058]FIG. 6 shows the structure of a downstream particulate filterdisposed in the emission control device of the first embodiment;

[0059]FIG. 7 shows the composition of exhaust gas from a Diesel engineincluding carbon-containing particulates and hydrocarbons;

[0060]FIG. 8 conceptually shows the spontaneous regenerating function ofan upstream particulate filter in the emission control device of theembodiment;

[0061]FIG. 9 conceptually shows a process of trapping carbon-containingparticulates included in the flow of exhaust gas by means of theupstream particulate filter;

[0062]FIG. 10 shows another layout where the upstream particulate filterand the downstream particulate filter are located proximate to eachother in another emission control device as one modified example of thefirst embodiment;

[0063]FIG. 11 shows still another layout where the downstreamparticulate filter is disposed at a joint of the flows of exhaust gas instill another emission control device as another modified example of thefirst embodiment;

[0064]FIG. 12 illustrates the construction of the Diesel engine with anemission control device of a second embodiment applied thereto;

[0065]FIG. 13 conceptually shows the active oxygen release function of adownstream particulate filter in the emission control device of thesecond embodiment;

[0066]FIG. 14 illustrates the construction of the Diesel engine with anemission control device of a third embodiment applied thereto;

[0067]FIG. 15 conceptually shows various modifications in the emissioncontrol device of the third embodiment;

[0068]FIG. 16 conceptually shows another modification in the emissioncontrol device of the third embodiment; and

[0069]FIG. 17 shows the principle of causing collision and adhesion ofthe carbon-containing particulates included in the flow of exhaust gasand thereby trapping the carbon-containing particulates.

BEST MODES OF CARRYING OUT THE INVENTION

[0070] With a view to further clarifying the functions and the effectsof the present invention, some modes of carrying out the presentinvention are discussed below in the following sequence:

[0071] A. First Embodiment

[0072] A-1. System Construction

[0073] A-1-1. Construction of Engine

[0074] A-1-2. Structure of Particulate Filter

[0075] A-2. Function of Regulating and Reducing Carbon-ContainingParticulates in First Embodiment

[0076] A-2-1. Control Function of Upstream Particulate Filter 100

[0077] A-2-2. Complementary Function of Downstream Particulate Filter200

[0078] A-3. Modifications

[0079] B. Second Embodiment

[0080] B-1. System Construction

[0081] B-1-1. Structure of Downstream Particulate Filter 300

[0082] B-1-2. Active Oxygen Release Function of Particulate Filter 300

[0083] B-2. Function of Regulating and Reducing Carbon-ContainingParticulates in Second Embodiment

[0084] C. Third Embodiment

[0085] C-1. Modifications

[0086] A. First Embodiment

[0087] A-1. System Construction

[0088] The following describes application of an emission control deviceof the present invention to a Diesel engine as one embodiment. Theemission control device of the present invention is applicable not onlyto Diesel engines but to gasoline engines where a fuel is directlyinjected into a cylinder for combustion and other internal combustionengines. The principle of the present invention is also applicable toany internal combustion engines for vehicles and ships as well asstationary internal combustion engines.

[0089] A-1-1. Construction of Engine

[0090]FIG. 1 schematically illustrates the construction of a Dieselengine 10 with an emission control device of a first embodiment appliedthereto. The Diesel engine 10 is a 4-cylinder engine and has fourcombustion chambers #1 through #4. The air is supplied to each of thecombustion chambers via an intake pipe 12, while a fuel is injected froma fuel injection valve 14 set in each combustion chamber. This leads tocombustion of the air and the fuel in the combustion chamber, andexhaust gas produced due to the combustion is discharged through anexhaust manifold 16 and an exhaust pipe 17 to the air.

[0091] A supercharger 20 is provided in the middle of the exhaust pipe17. The supercharger 20 has a turbine 21 located in the exhaust pipe 17,a compressor 22 set in the intake pipe 12, and a shaft 23 connecting theturbine 21 with the compressor 22. The flow of exhaust gas dischargedfrom the combustion chamber rotates the turbine 21 of the supercharger20, so as to drive, via the shaft 23, a compressor 22, which compressesthe air and supplies the compressed air to each combustion chamber. Anair cleaner 26 is arranged upstream of the compressor 22. The compressor22 compresses the intake air through the air cleaner 26 and feeds thecompressed air into each combustion chamber. Since the air compressed bythe compressor 22 has the raised temperature, an inter cooler 24 forcooling down the air is disposed downstream of the compressor 22. Thecompressed air may thus be cooled down by the inter cooler 24 andsubsequently fed into the combustion chamber.

[0092] A particulate filter 100 is disposed in each of the combustionchambers #1 through #4 in the exhaust manifold 16, and a particulatefilter 200 is disposed in the exhaust pipe 17. Namely the particulatefilters 100 are arranged upstream of the turbine 21, whereas theparticulate filter 200 is arranged downstream of the turbine 21. Each ofthe particulate filters 100 disposed upstream of the turbine 21 trapsparticulates and hydrocarbon compounds included in the flow of exhaustgas, while utilizing the reaction heat of the hydrocarbon compounds tomake the trapped particulates subjected to combustion in the exhaust gasof relatively low temperature. The particulate filter 200 disposeddownstream of the turbine 21 traps the remaining carbon-containingparticulates passing through the upstream particulate filters 100. Theparticulate filters 100 and 200 will be discussed later in detail.

[0093] The particulate filter 100 may be disposed in the exhaust pipe 17upstream of the turbine 21, instead of in the exhaust manifold 16. Inthe structure where the particulate filters 100 are disposed in theexhaust manifold 16, the dynamic pressure of the exhaust gas ejectedfrom the combustion chambers is effectively convertible into heat by thefunction of the particulate filters 100. This desirably acceleratescombustion of the carbon-containing particulates trapped on the filters.In the structure where the particulate filter 100 is disposed in theexhaust pipe 17 upstream of the turbine 21, on the other hand, the lessspatial restriction desirably allows application of a large-capacityfilter.

[0094] A fuel supply pump 18 and the fuel injection valve 14 undercontrol of an engine control ECU 30 function to inject an appropriatequantity of fuel into each combustion chamber at an adequate timing. Theengine control ECU 30 detects driving conditions of the engine includingengine speed Ne and an accelerator opening θac, and adequately regulatesthe fuel supply pump 18 and the fuel injection valve 14 according to thedetected driving conditions.

[0095] A-1-2. Structure of Particulate Filter

[0096] The description first regards the particulate filter 100 disposedin each of combustion chambers upstream of the turbine 21, and then theparticulate filter 200 disposed downstream of the turbine 21.

[0097] (1) Structure of Upstream Particulate Filter 100

[0098]FIG. 2 is a perspective view illustrating the appearance of theparticulate filter 100 disposed upstream of the turbine 21. With a viewto better understanding, part of the cross section is enlarged to showthe internal structure. The particulate filter 100 includes acylindrical case 102 and an element 104 that is inserted in the case 102and has the outer circumference welded to the case 102. The element 104has a rolled cylindrical structure, in which a non-woven fabric 106 of aheat-resistant metal and a corrugated sheet 108 of a heat-resistantmetal in piles are rolled up on a core 110. The element 104 used in theparticulate filter 100 of the embodiment has the outer diameter ofapproximately 55 mm and the length of approximately 40 mm. Thesedimensions can appropriately be varied according to the displacement ofthe Diesel engine and the inner diameter of the exhaust manifold 16 orthe exhaust pipe 17.

[0099] The non-woven fabric 106 is rolled up with the corrugated sheet108, such that adjoining layers of the non-woven fabric 106 are kept atfixed intervals by means of the corrugated sheet 108. A large number ofpathways along the axis of the core 110 are accordingly formed betweenthe non-woven fabric 106 and the corrugated sheet 108. Sealing plates112 are welded to both ends of the element 104. The sealing plates 112alternately close the pathways formed between the non-woven fabric 106and the corrugated sheet 108, so as to define the construction thatallows the flow of the exhaust gas to pass through the non-woven fabric106. The function of the sealing plates 112 to define the constructionthat allows passage of the exhaust gas through the non-woven fabric 106is discussed below with reference to FIG. 3.

[0100]FIG. 3 conceptually illustrates the sectional structure of theparticulate filter 100. For simplicity of illustration, the corrugatedsheet 108 is omitted from the illustration of FIG. 3. As clearly shown,the sealing plates 112 alternatively close the pathways formed betweenthe adjoining layers of the non-woven fabric 106 kept at fixedintervals. The flow of the exhaust gas from the left side of the drawingas shown by the hatched arrows in FIG. 3 enters the pathways that arenot closed by the sealing plates 112. The outlets of these pathways are,however, closed by the sealing plates 112. The flow of the exhaust gasaccordingly penetrates the non-woven fabric 106 defining the side facesof the pathways and goes to the pathways having the non-closed outletsas shown by the thick arrows. As the flow of the exhaust gas passesthrough the non-woven fabric 106, carbon-containing particulates likesoot included in the exhaust gas are trapped by the non-woven fabric106.

[0101] A non-woven fabric that is made of a heat-resistant iron alloyand has dimensions of predetermined ranges as shown in FIG. 4 is appliedfor the non-woven fabric 106. The non-woven fabric 106 can thusdispersedly trap the carbon-containing particulates and the hydrocarboncompounds in such a manner that brings the respective particulates andhydrocarbon compounds in contact with oxygen in the exhaust gas.Trapping the particulates in the three-dimensionally dispersed mannercauses spontaneous combustion when the total amount of the trappedparticulates reaches a certain level. The mechanism of dispersedlytrapping the carbon-containing particulates and the hydrocarboncompounds, as well as the mechanism of spontaneous combustion of thedispersedly trapped carbon-containing particulates will be discussedlater.

[0102] The ‘mean fiber diameter’ in the table of FIG. 4 represents amean diameter of metal fibers constituting the non-woven fabric. Themetal non-woven fabric is made of countless metal fibers tangled in acomplicated manner. Intricately divaricating three-dimensional pathwaysare formed between the metal fibers. The ‘mean pore diameter’ is anindex representing the cross section of the pathways formed between themetal fibers. In the specification hereof, the mean pore diameterrepresents the mean value of the pore diameter measured according to theWashburn's equation, and is the pore diameter having the accumulatedpore volume of 50%. The numerical value of the mean pore diameter isvaried in measurement of another known method. The simplest way observesthe surface or the cross section of the metal non-woven fabric with amicroscope for measurement.

[0103] The dimensions of the non-woven fabric shown in FIG. 4 are onlyillustrative and are not restrictive in any sense. Although the metalnon-woven fabric of the heat-resistant iron alloy is used in thisembodiment, the metal non-woven fabric may be made of any other knownheat-resistant metal.

[0104] In the structure of this embodiment, the sealing plates 112 arewelded to both ends of the element 104. One possible modification is astructure without the sealing plates 112.

[0105]FIG. 5 is a sectional view illustrating the modified structure ofthe particulate filter 100 in which the element does not have thesealing plates. For clarity of illustration, the corrugated sheet 108 isomitted from the illustration of FIG. 5. In the structure of theembodiment shown in FIG. 3, the sealing plates 112 are alternatelywelded to both ends of the non-woven fabric 106. Instead of welding thesealing plates, the adjoining layers of the non-woven fabric are weldedto each other at ends 113 in the modified structure shown in FIG. 5.Such modified arrangement does not require the sealing plates 112 andthus simplifies the structure of the particulate filter 100.

[0106] (2) Structure of Downstream Particulate Filter 200

[0107]FIG. 6 shows the structure of the particulate filter 200 disposeddownstream of the turbine 21. FIG. 6(a) is a front view of theparticulate filter 200 seen from the inflow side of the exhaust gas, andFIG. 6(b) is a side sectional view. As illustrated, the particulatefilter 200 arranged downstream of the turbine 21 is a cordierite ceramicfilter of honeycomb structure. A large number of pathways 202 are formedin the particulate filter 200 of honeycomb structure to allow passage ofthe exhaust gas. Fillers 204 are alternately attached to upstream endsor downstream ends of these pathways. The fillers 204 are expressed byhatching in FIG. 6.

[0108] The flow of exhaust gas from the left side of FIG. 6(b) entersthe particulate filter 200 through the pathways 202 without the fillers204 on their upstream ends. The downstream ends of these pathways are,however, closed by the fillers 204. As shown by the thick arrows in FIG.6(b), the flow of the exhaust gas passes through bulkheads 206 of thepathways 202 to the pathways 202 without the fillers 204 on theirdownstream ends. Cordierite has the porous structure formed in theprocess of calcination. As the flow of exhaust gas passes through theporous structure of the bulkheads 206, the porous structure traps thecarbon-containing particulates in the exhaust gas.

[0109] The particulate filter 200 downstream of the turbine 21 is notrestricted to the ceramic filter. Any known heat-resistant filter may beapplicable as long as it has a pore diameter distribution equivalent toor smaller than the pore diameter of the particulate filter 100 upstreamof the turbine 21.

[0110] A-2. Function of Regulating and Reducing Carbon-ContainingParticulates in First Embodiment

[0111] The following describes the function of regulating and reducingthe carbon-containing particulates in the emission control device of thefirst embodiment where the particulate filters 100 are disposed upstreamof the turbine 21 and the particulate filter 200 is disposed downstreamof the turbine 21. In the emission control device of the firstembodiment, the upstream particulate filters 100 and the downstreamparticulate filter 200 function in a complementary manner, so that thecarbon-containing particulates included in the exhaust gas areefficiently controlled. The following description first regards the‘spontaneous regenerating function’ of the upstream particulate filters100 and then the function of the complementary downstream particulatefilter 200.

[0112] A-2-1. Control Function of Upstream Particulate Filter 100

[0113] The non-woven fabric 106 of the particulate filter 100 is theheat-resistant non-woven fabric having the dimensions of thepredetermined ranges shown in FIG. 4. The carbon-containing particulatesand the hydrocarbon compounds included in the exhaust gas aredispersedly trapped inside the non-woven fabric 106 to come into contactwith oxygen in the exhaust gas. Trapping the particulates in thedispersed manner causes spontaneous combustion when the total amount ofthe trapped particulates reaches a certain level, even if thetemperature of the exhaust gas is lower than the combustible temperatureof the carbon-containing particulates. This ensures effective combustionof the carbon-containing particulates trapped on the filter. Thisfunction of the particulate filter 100 is referred to as the‘spontaneous regenerating function’ in this specification. The mechanismof the ‘spontaneous regenerating function’ has not been fullyelucidated, but the estimated mechanism is briefly explained below.

[0114] As is known in the art, the exhaust gas from the Diesel engineincludes the carbon-containing particulates and the hydrocarboncompounds at a ratio shown in FIG. 7. Roughly speaking, the exhaust gasincludes practically similar fractions of the particulates like soot,the fuel-attributed hydrocarbon compounds, and the lubricantoil-attributed hydrocarbon compounds. The carbon-containing particulateslike soot are not subjected to combustion at temperatures of lower than550° C. even in the atmosphere of the oxygen-containing exhaust gas. Itis expected, on the other hand, that the fuel-attributed hydrocarboncompounds and the lubricant oil-attributed hydrocarbon compounds aresubjected to the oxidation reaction even at temperature of lower than550° C. under the condition of a sufficient supply of oxygen.

[0115] The particulate filter 100 of the embodiment traps theparticulates and the hydrocarbon compounds in the exhaust gas in athree-dimensionally dispersed manner inside the non-woven fabric. Thetrapped hydrocarbon compounds accordingly receive a sufficient supply ofoxygen in the exhaust gas and starts a gentle oxidation reaction(exothermic reaction) with the heat of the exhaust gas. This graduallyraises the filter temperature and causes accumulation of reactiveintermediate products. When the total amount of the particulates and thehydrocarbon compounds trapped on the filter approaches a certain level,the filter temperature exceeds 550° C. and causes combustion of theparticulates and the hydrocarbon compounds trapped on the filter.

[0116]FIG. 8 conceptually shows spontaneous regeneration of theparticulate filter 100 of the embodiment. FIG. 8(a) schematically showsthe particulate filter 100 disposed in the exhaust pipe 16 of the Dieselengine 10. FIG. 8(b) shows the measurement results of differentialpressure dP before and after the filter, temperature Tg of the exhaustgas flown into the filter, and filter temperature Tf, while the Dieselengine 10 is driven under fixed conditions. The differential pressure dPbefore and after the filter is measured with pressure sensors 64 and 66arranged upstream and downstream of the filter.

[0117] When the Diesel engine 10 starts driving, the exhaust gastemperature Tg and the filter temperature Tf immediately rise to astationary level. Although the filter temperature Tf is actually higherthan the exhaust gas temperature Tg, for simplicity of explanation, itis assumed here that the two temperatures have no significantdifference.

[0118] In the case of a new particulate filter 100, the differentialpressure dP before and after the filter gradually increases and iseventually stabilized at a fixed value. The stable differential pressurebefore and after the filter at the fixed value is ascribed to the factthat the particulate filter 100 of the embodiment traps the particulatesin the exhaust gas not only on the filter surface but inside the filterin a three-dimensional manner. The value of the stabilized differentialpressure is varied mainly by the design dimensions of the filter, but istypically three to four times of the initial differential pressure. Forconvenience of explanation, the time period between the start ofoperation of the Diesel engine 10 and the stabilization of thedifferential pressure before and after the filter is called the ‘firstterm’.

[0119] When the Diesel engine 10 continues driving after stabilizationof the differential pressure before and after the filter, the filtertemperature Tf starts a gentle rise, whereas the exhaust gas temperatureTg is not significantly varied. The deviation of the filter temperatureTf from the exhaust gas temperature Tg gradually increases, and thefilter temperature Tf eventually reaches about 550° C. The differentialpressure dP before and after the filter tends to slightly increase, dueto the trapped particulates like soot and the hydrocarbon compounds onthe filter, although the level of increase may be insignificant.

[0120] When the filter temperature Tf rises to 550° C., the soot and theother particulates trapped on the filter start combustion. On combustionof all the trapped particulates, the filter temperature Tf isimmediately lowered to the level of the exhaust gas temperature Tg. Inthe case where the increase in differential pressure dP before and afterthe filter, due to trapping the soot and the other particulates in theexhaust gas, is detectable, a decrease in differential pressure dP, dueto combustion of the soot and the other particulates trapped on thefilter, is measurable. The time period subsequent to the first term whenthe filter temperature Tf is gradually deviated from the exhaust gastemperature Tg and again dropped to the exhaust gas temperature Tg iscalled the ‘second term’. The first term is appreciably shorter than thesecond term. For clarity of illustration, the first term illustrated inFIG. 8 is longer than the actual length relative to the second term.

[0121] The filter temperature Tf is once lowered to the level of theexhaust gas temperature Tg on completion of combustion of the soot andthe other particulates trapped on the filter, but again rises to 550° C.to start combustion of the trapped soot and the other particulates.Namely the filter is kept in the state of the second term to repeattrapping and combustion of the soot and the other particulates includedin the exhaust gas. This is the first state of the spontaneousregenerating function of the particulate filter 100.

[0122] The second state of the spontaneous regenerating function appearsunder the condition of the higher exhaust gas temperature Tg. FIG. 8(c)shows variations in filter temperature Tf and in differential pressuredP before and after the filter when the Diesel engine 10 is driven underthe condition of a litter higher exhaust gas temperature (typically,higher by 50° C.) than the condition of FIG. 8(b). This phenomenon isnot restricted to the case of varying the exhaust gas temperature.Similar results are obtained when the density of soot is varied to be alittle higher than the condition of FIG. 8(b).

[0123] Under the condition of the higher exhaust gas temperature Tg, thefilter temperature Tf is not lowered to the level of the exhaust gastemperature Tg but is stabilized at a higher level after the second termas shown in FIG. 8(c). The time period subsequent to the second termwhen the filter temperature Tf is stabilized at a higher level than theexhaust gas temperature Tg is referred to as the ‘third term’. In thethird term, it is expected that trapping and combustion of the soot andthe other particulates are locally repeated or that trapping andcombustion proceed simultaneously at an identical location. In any case,the differential pressure dP before and after the filter is kept at asubstantially fixed value in the third term as shown in FIG. 8(c). Inthe second state of the natural regenerating function, trapping andcombustion of the particulates are carried out in parallel.

[0124] As described above, the particulate filter 100 disposed upstreamof the turbine 21 has the non-woven fabric of the predetermineddimensions and dispersedly traps the carbon-containing particulates andthe hydrocarbon compounds in the exhaust gas. The dispersedly trappedparticulates are subjected to spontaneous combustion without anyspecific operations. The particulate filter 100 traps the soot and theother particulates in the dispersed manner, since the non-woven fabricactively takes in and traps the particulates according to somemechanism. The estimated trapping mechanism is described briefly.

[0125]FIG. 9 conceptually shows the cross sectional structure of anon-woven fabric of a heat-resistant metal. The hatched circles in thedrawing respectively represent the cross sections of fibers of thenon-woven fabric. The non-woven fabric is composed of countless fiberstangled intricately and has numerous three-dimensional pathwaysconnecting with one another in a complicated manner.

[0126]FIG. 9(a) conceptually shows the cross sectional structure of anew non-woven fabric. It is here assumed that the exhaust gas flowsdown. Because of the variation in distribution, of fibers, opening ofvarious sizes are formed on the surface of the non-woven fabric. Eventhe small opening is sufficiently large for the gas molecules in theexhaust gas. The flow of the exhaust gas thus passes through the wholesurface of the non-woven fabric in a practically uniform manner. In thedrawing of FIG. 9(a), the flows of the exhaust gas between the fibers ofthe non-woven fabric are schematically expressed by the thick arrows.

[0127] As the flow of the exhaust gas passes through the non-wovenfabric, the particulates like soot included in the exhaust gas aretrapped between the fibers and gradually clog the openings on thesurface of the non-woven fabric. The small openings on the surface ofthe non-woven fabric are clogged with the particulates like soot, andthe flows of the exhaust gas go to the non-clogged but remaining,relatively large openings as shown in FIG. 9(b). The flows of theexhaust gas passing through the non-woven fabric accordingly meettogether to the flows from the non-clogged but remaining, relativelylarge openings on the surface. In the drawing of FIG. 9(b), theparticulates like soot are schematically expressed by the small closedcircles.

[0128] The integrated flow of the exhaust gas increases the flow rateand causes a significant pressure gradient in the pathway. Thisphenomenon may be explained by the collision of the flow against thefibers of the non-woven fabric to produce a large pressure. As mentionedpreviously, the pathways formed inside the non-woven fabric communicatewith one another in an intricate manner. The higher pressure of theintegrated flow in the pathway causes the flow to immediately branch offto the other pathways. The differential pressure before and after thenon-woven fabric thus does not increase to or above a preset level butis kept in a fixed range.

[0129]FIG. 9(c) conceptually shows the main stream branching off to theother pathways. As the flow of the exhaust gas branches off in thenon-woven fabric, the carbon-containing particulates like soot includedin the exhaust gas are trapped by the whole area of the non-wovenfabric. Even if a certain place in the non-woven fabric is clogged withsoot, the three-dimensional connection of the pathways allows the flowto immediately branch off to the other pathways. Namely even when acertain place in the non-woven fabric is clogged with soot and the otherparticulates, the flow path of the exhaust gas is automatically changedto new pathways. This arrangement ensures dispersed trapping of the sootand the other particulates.

[0130] As described above, the particulate filter 100 disposed upstreamof the turbine 21 has the spontaneous regenerating function, and causesthe trapped carbon-containing particulates and the hydrocarbon compoundsin the exhaust gas to be subjected to spontaneous combustion without anyspecial operations. The particulate filter 100 of this embodiment usesthe metal non-woven fabric 106 to trap the particulates in the exhaustgas. The particulate filter 100 is, however, not restricted to the metalnon-woven fabric but may be a ceramic filter like a cordierite honeycombfilter. The ceramic filter having the equivalent pore diameterdistribution can exert the spontaneous regenerating function equivalentto that of the filter of the embodiment.

[0131] A-2-2. Complementary Function of Downstream Particulate Filter200

[0132] The following describes the complementary function of theparticulate filter 200 disposed downstream of the turbine 21 tocomplement the upstream particulate filter 100. As described above, theparticulate filter 100 disposed upstream of the turbine 21 traps thecarbon-containing particulates and the hydrocarbon compounds in theexhaust gas in a dispersed manner inside the non-woven fabric having thedimensions of the predetermined ranges shown in FIG. 4.

[0133] When the pore diameter of the non-woven fabric is excessivelyreduced to enhance the trapping rate of the particulates by the upstreamparticulate filter 100, the non-woven fabric has difficulty in activelytaking in and dispersedly trapping the particulates as described abovewith reference to FIG. 9. When the upstream particulate filters 100 areused alone, a trace amount of the carbon-containing particulatesincluded in the exhaust gas may pass through the filters. The downstreamparticulate filter 200 traps the carbon-containing particulates passingthrough the upstream particulate filters 100, thus significantlyreducing the total quantity of the particulates released to the air.

[0134] Most of the carbon-containing particulates discharged from theDiesel engine 10 are trapped by the upstream particulate filters 100having the spontaneous regenerating function, and the downstreamparticulate filter 200 traps only a trace amount of thecarbon-containing particulates passing through the upstream filters.There is accordingly only a little amount of particulates accumulated onthe downstream particulate filter 200. It is thus required to raise thetemperature of the exhaust gas emitted from the Diesel engine 10 at rareintervals for combustion of the carbon-containing particulatesaccumulated on the downstream filter. A diversity of known techniquesmay be applied to raise the exhaust gas temperature. One applicabletechnique is intake air restriction that uses a valve in the intake pipe12 of the Diesel engine 10 and narrows down the valve to raise theemission temperature. Another applicable technique delays the fuelinjection timing behind an appropriate timing to raise the emissiontemperature. Although any technique naturally lowers the fuelconsumption efficiency, the temperature rise at rare intervals does notsignificantly worsen the fuel consumption efficiency.

[0135] The downstream particulate filter 200 traps only a trace amountof the carbon-containing particulates and is not filled with theparticulates even after use of a relatively long time. Replacement ofthe whole particulate filter 200 every when it is filled with theparticulates is required only at rare intervals. Namely no large costnor long time is required for the replacement. The particulate filter200 may be mounted at a position that allows easy access forreplacement, for example, at a certain position under the floor of thevehicle. An alternative application uses the particulate filter 200 oflarge dimensions, so that no replacement of the filter is required inthe normal use conditions.

[0136] Using the downstream particulate filter 200 desirably enhancesthe degree of freedom in setting the upstream particulate filter. Forthe effective spontaneous regenerating function, it is effective tolocate the particulate filter 100 on the upper stream side, for example,to locate the particulate filter 100 between the combustion chambers ofthe Diesel engine and the supercharger 20 if possible. The area closerto the combustion chamber may, however, not receive the particulatefilter 100 of sufficient dimensions, because of the spatial restriction.In such cases, the presence of the downstream particulate filter 200 tocomplement the upstream particulate filter 100 sufficiently reduces thetotal quantity of the carbon-containing particulates released to theair.

[0137] Providing the downstream particulate filter 200 to complement theupstream particulate filter 100 effectively prevents the upstreamparticulate filter 100 from being clogged with ash. Here the ashrepresents metal components like Ca, Mg, and Zn included in additives ofengine oil and combined with sulfur in the fuel to form sulfates anddeposit as ash content. The metal sulfates are thermally very stable.Rising the exhaust gas temperature accordingly can not cause combustionof the ash accumulated on the filter. In the case where the fuel has alarge sulfur content, it is desirable not to trap the ash but to makethe ash penetrate the filter. Passage of the ash naturally leads topassage of the carbon-containing particulates through the filter. In theemission control device of the embodiment, the presence of thedownstream particulate filter 200 effectively prevents an increase inquantity of the particulates released to the air, even in the case of anincrease in quantity of the particulates passing through the upstreamparticulate filter 100. Namely the upstream particulate filter 100 maybe constructed to allow passage of the ash, owning to the presence ofthe complementary downstream particulate filter 200. This arrangementeffectively prevents the upstream particulate filter 100 from beingclogged with the ash.

[0138] The ash penetrating the upstream filter reaches the downstreamparticulate filter 200. Because of the following reason, there is arelatively small amount of the ash accumulated on the downstream filter.The ash itself is a dry substance, but the hydrocarbon compoundsincluded in the exhaust gas function like an adhesive and adhere to thenon-woven fabric. The ash is successively accumulated on the hydrocarboncompounds, grows on the non-woven fabric, and eventually clogs thefilter. As described previously, the upstream particulate filter 100traps the carbon-containing particulates and the hydrocarbon compoundsfor combustion. There is accordingly only little amount of thehydrocarbon compounds functioning as the adhesive on the downstreamparticulate filter 200. This desirably prevents accumulation of the ashon the downstream particulate filter 200. Namely the upstreamparticulate filter 100 effectively works to prevent accumulation of theash on the downstream particulate filter 200.

[0139] The arrangement of the particulate filter 200 downstream of thesupercharger 20 as shown in FIG. 1 further prevents accumulation of theash. The turbine 21 of the supercharger 20 generally rotates at a highspeed of not less than 10,000 rotations per minute. The ash isaccordingly crushed down to finer particles when passing through theturbine 21. The finer particles of the ash are not easily trapped by thefilter and do not accumulate on the filter.

[0140] Because of the reasons discussed above, only little amount of theash is accumulated on the downstream filter. The combination of theupstream particulate filter 100 with the complementary downstreamparticulate filter 200 effectively prevents the filters from beingclogged with the ash. The downstream particulate filter 200 may beclogged with the ash after a long-time use. In such cases, the wholedownstream particulate filter may be replaced. Replacement of the filterat rare intervals does not require so much cost or labor. On theassumption of replacement of the particulate filter 200, combustion ofthe carbon-containing particulates trapped by the downstream filter maybe omitted. This desirably simplifies the whole system.

[0141] A-3. Modifications

[0142] In the emission control device of the first embodiment describedabove, a cordierite ceramic filter is applied for the downstreamparticulate filter 200. Like the upstream filter, the downstreamparticulate filter 200 may have the spontaneous regenerating function.In a modified example discussed below, a spontaneous regenerating filterhaving a slightly smaller setting of the pore diameter distribution thanthat of the upstream particulate filter 100 is applied for theparticulate filter 200 disposed downstream of the turbine 21.

[0143] In the emission control device of the modified example, thecarbon-containing particulates passing through the upstream particulatefilter 100 are trapped by the downstream particulate filter 200. Thecarbon-containing particulates reaching the downstream particulatefilter 200 are fine particles passing through the upstream particulatefilter 100. The slightly smaller setting of the pore diameterdistribution of the downstream particulate filter 200 than that of theupstream particulate filter 100 enables the particulates to bedispersedly trapped inside the filter according to the mechanismdescribed above with reference to FIG. 8.

[0144] The hydrocarbon compounds and oxygen in the exhaust gas areconsumed by the upstream particulate filter 100, and there is a lesssupply to the downstream particulate filter 200 compared with theupstream filter. The concentration of oxygen in the exhaust gas issignificantly varied according to the driving conditions of the Dieselengine. In the actual state, the driving conditions are drasticallyvaried, so that a sufficient quantity of oxygen may be supplied to thedownstream particulate filter. It is also relatively easy to raise theconcentration of the hydrocarbon compounds in the exhaust gas bymatching the driving state of the engine. This also allows a sufficientquantity of the hydrocarbon compounds to be supplied to the downstreamparticulate filter 200.

[0145] The particulate filter 200, which is disposed downstream of theupstream particulate filter 100 and has the slightly smaller setting ofthe pore diameter distribution and the spontaneous regeneratingfunction, traps the carbon-containing particulates passing through theupstream filter. This combination significantly reduces the totalquantity of the particulates released to the air. As described above,both the upstream particulate filter 100 and the downstream particulatefilter 200 have the spontaneous regenerating function and do not requireany special treatment for combustion of the carbon-containingparticulates trapped on the respective filters.

[0146] In this modified example, the downstream particulate filters 200may be disposed immediately after the upstream particulate filters 100as shown in FIG. 10. Such layout enables the exhaust gas of hightemperature to be flown into the downstream particulate filters 200.This arrangement effectively utilizes the spontaneous regeneratingfunction of the downstream particulate filters 200 for combustion of thetrapped carbon-containing particulates.

[0147] In another layout shown in FIG. 11, the upstream particulatefilter 100 is provided in each of multiple combustion chambers, whereasthe downstream particulate filters 200 are provided at joints of theexhaust pathways from the multiple combustion chambers. The downstreamparticulate filters 200 of relatively large dimensions may be used inthis arrangement.

[0148] B. Second Embodiment

[0149] B-1. System Construction

[0150]FIG. 12 conceptually illustrates an emission control device of asecond embodiment applied to the Diesel engine 10. Main differences ofthe emission control device of the second embodiment from the firstembodiment are that a cordierite particulate filter 300 with an activeoxygen release agent carried thereon is used in place of the downstreamparticulate filter 200 of the first embodiment and that a throttle valve60 is disposed in the intake pipe 12. The throttle valve 60 is driven bymeans of a step motor 62. The throttle valve 60 is fully open in thenormal driving conditions, but is closed to a preset opening undercontrol of the engine control ECU 30 in response to requirement ofdecreasing the concentration of oxygen in the exhaust gas. The concreteprocedure of such control will be discussed later.

[0151] The active oxygen release agent carried on the particulate filter300 of the second embodiment takes in and keeps oxygen in the presenceof excess oxygen in its atmosphere, while releasing the intake oxygen inthe form of active oxygen with a decrease in concentration of oxygen inthe atmosphere. The active oxygen is much more reactive than thestandard oxygen. The active oxygen can thus be utilized for easycombustion of the carbon-containing particulates trapped on theparticulate filter 300.

[0152] In the emission control device of the second embodiment, theupstream particulate filters 100 having the spontaneous regeneratingfunction and the downstream particulate filter 300 with the activeoxygen release agent carried thereon function complimentarily toeffectively reduce the carbon-containing particulates included in theexhaust gas.

[0153] B-1-1. Structure of Downstream Particulate Filter 300

[0154] Like the downstream particulate filter 200 of the firstembodiment, the particulate filter 300 with the active oxygen releaseagent carried thereon is a ceramic filter of honeycomb structure. Thestructure of the particulate filter 300 of the second embodiment isbriefly described with reference to FIG. 6, which shows the structure ofthe downstream particulate filter 200 of the first embodiment.

[0155] As shown in FIG. 6, the particulate filter 300 of the secondembodiment also has the honeycomb structure and has a large number ofpathways 202 formed therein. Fillers 204 are alternately attached toupstream ends or downstream ends of these pathways 202.

[0156] The flow of exhaust gas from the left side of FIG. 6(b) entersthe particulate filter 300 through the pathways 202 without the fillers204 on their upstream ends and passes through bulkheads 206 of thepathways 202 to the pathways 202 without the fillers 204 on theirdownstream ends as shown by the thick arrows. Cordierite has the porousstructure formed in the process of calcination. As the flow of exhaustgas passes through the porous structure of the bulkheads 206, the porousstructure traps the carbon-containing particulates in the exhaust gas.

[0157] A base material layer mainly composed of alumina is formed on thesurface of the porous structure of the cordierite bulkheads 206, and anoble metal catalyst and the active oxygen release agent are carried onthe base material layer. Platinum Pt is mainly used as the noble metalcatalyst, although another metal having oxidation activity likepalladium Pd may be applied. Typical examples of the active oxygenrelease agent include alkaline metals, such as potassium K, sodium Na,lithium Li, cesium Cs, and rubidium Rb, alkaline earth metals, such asbarium Ba, calcium Ca, and strontium Sr, rare earth elements, such aslanthanum La, yttrium Y, and cerium Ce, and transition metals.Especially the alkaline metals and the alkaline earth metals having thehigher ionicity than calcium Ca, that is, potassium K, lithium Li,cesium Cs, rubidium Rb, barium Ba, and strontium Sr are preferably usedfor the active oxygen release agent.

[0158] B-1-2. Active Oxygen Release Function of Particulate Filter 300

[0159] The following briefly describes how the particulate filter 300 ofthe second embodiment with the active oxygen release agent carriedthereon takes in excess oxygen in the exhaust gas and releases theintake oxygen in the form of active oxygen.

[0160]FIG. 13 conceptually shows the surface of the porous structure ofthe particulate filter 300. An active oxygen release agent 302 likepotassium K or barium Ba and a noble metal catalyst 304 like platinum Ptare carried on the surface of the porous structure of the particulatefilter 300. The noble metal catalyst 304 like platinum Pt is in the formof particles having the particle diameter of not greater than 1 μm andis homogeneously dispersed on the active oxygen release agent 302.

[0161]FIG. 13(a) conceptually shows a process that the active oxygenrelease agent 302 takes in excess oxygen included in the exhaust gas.The exhaust gas includes nitrogen oxides generated through combustion.The nitrogen oxides are included mostly in the form of nitrogen monoxideNO, and are thus expressed as nitrogen monoxide NO in FIG. 13. Nitrogenmonoxide NO is a polar molecule, and NO in the exhaust gas is thusquickly adsorbed on platinum Pt. Pt has relatively high oxidationactivity, so that NO reacts with oxygen in the exhaust gas on Pt to beoxidized to nitrate ion NO₃— via nitrite ion NO₂— and is taken in theform of nitrates into the active oxygen release agent 302. The nitrateion NO₃— on Pt shifts to the active oxygen release agent 302 by aphenomenon called ‘spillover’. The ‘spillover’ phenomenon is that theadsorbed molecules actively move around on the catalyst. The particlesof Pt or another metal are dispersed on the catalyst in a homogeneousmanner, but are still localized in the molecular level. The spilloverphenomenon causes the adsorbed molecules to actively move around on thesurface of the catalyst. The whole surface accordingly functions as thecatalyst. In the presence of excess oxygen in the exhaust gas, NO isoxidized on Pt and is shifted to the active oxygen release agent 302 bythe spillover phenomenon to be stored in the form of nitrates withexcess oxygen. In the above description, it is assumed that carbonmonoxide adsorbed on Pt is all oxidized to the nitrate ion NO₃—. In theactual state, however, carbon monoxide may not be all oxidized to thenitrate ion NO₃—, but may be partly stored in the form of nitrites.

[0162]FIG. 13(b) conceptually shows a process that the active oxygenrelease agent 302 releases oxygen stored with NO in the form of activeoxygen with a decrease in concentration of oxygen in the exhaust gas.The exhaust gas includes reducing substances, such as hydrocarboncompounds and carbon monoxide. The carbon-containing particulates likesoot also function as the reducing substances. In the drawing of FIG.13(b), the hydrocarbon compounds are expressed as HC, and thecarbon-containing particulates like soot are expressed as carbon C. Asmentioned previously, Pt has the high oxidation activity and acceleratesoxidation of such reducing substances in the presence of oxygen in theexhaust gas to carbon dioxide CO₂ and water.

[0163] In the absence of a sufficient quantity of oxygen relative to thereducing substances, however, as shown in FIG. 13(b), Pt decomposes thenitrate ion NO₃— or nitrite ion NO₂— stored in the active oxygen releaseagent 302 and oxidizes the reducing substances with active oxygenproduced in the course of decomposition. This phenomenon is explainedwith reference to FIG. 13(b). The nitrate ion NO₃— (or nitrite ion NO₂—)stored in the active oxygen release agent 302 shifts onto Pt by thespillover phenomenon. The electron cloud of the nitrate ion NO₃— isattracted by Pt to be localized. Such localization weakens the chemicalbonding of nitrogen atom to oxygen atoms in the nitrate ion NO₃—. In thedrawing of FIG. 13(b), the nitrate ion is expressed as ‘N+3.O’, whichschematically represents the weak bonding of nitrogen atom to oxygenatoms. Action of the reducing substances in this state cuts the bondingof nitrogen atom to oxygen atom and produces active oxygen. Activeoxygen is extremely reactive and quickly reacts with the hydrocarboncompounds, carbon monoxide, and the carbon-containing particulates likesoot in the exhaust gas so as to convert them into carbon dioxide CO₂and water.

[0164] The particulate filter 300 of the second embodiment takes inoxygen in the presence of excess oxygen in the exhaust gas, whilereleasing the intake oxygen in the form of active oxygen in the absenceof oxygen. Conversion of the carbon-containing particulates with theactive oxygen into carbon monoxide and water effectively reduces thecarbon-containing particulates trapped on the filter.

[0165] B-2. Function of Regulating and Reducing Carbon-ContainingParticulates in Second Embodiment

[0166] As described above, the emission control device of the secondembodiment uses the combination of the upstream particulate filters 100having the spontaneous regenerating function with the downstreamparticulate filter 300 having the active oxygen release function. Thecombination complements the respective functions and effectively reducesthe carbon-containing particulates included in the exhaust gas.

[0167] In the normal driving state, excess oxygen is present in theexhaust gas emitted from the Diesel engine. The carbon-containingparticulates and the hydrocarbon compounds in the exhaust gas aretrapped in a dispersed manner by the upstream particulate filters 100.As described previously, the particulate filters 100 have thespontaneous regenerating function to slowly oxidize the trappedhydrocarbon compounds with oxygen in the exhaust gas for combustion ofthe trapped carbon-containing particulates.

[0168] Under the normal driving conditions, the exhaust gas containsexcess oxygen, so that the downstream particulate filter 300 takesoxygen in the exhaust gas together with nitrogen oxides into the activeoxygen release agent in the form of nitrates (or nitrites). Thedownstream particulate filter 300 also traps a trace amount of thecarbon-containing particulates passing through the upstream particulatefilters 100.

[0169] In the emission control device of the second embodiment, theupstream particulate filters 100 trap most of the carbon-containingparticulates. There is thus practically no possibility that a largequantity of the particulates are accumulated on the downstreamparticulate filter 300. If the exhaust gas contains a large quantity ofthe carbon-containing particulates like soot, the noble metal, such asPt, having the catalytic action is covered with soot. This preventsoxygen and the nitrogen oxides in the exhaust gas from being taken inthe form of nitrates. This state is called carbon poisoning. The sootcovering over Pt is initially amorphous carbon but is eventuallydeformed to stable graphite, which may not be successfully burned upwith the heat of the exhaust gas. In the emission control device of thesecond embodiment, however, the upstream particulate filters 100 trapmost of the carbon-containing particulates, and the downstreamparticulate filter 100 is required to trap only a trace amount of theparticulates passing through the upstream filters and is thus free fromcarbon poisoning.

[0170] In the emission control device of the second embodiment, thecarbon-containing particulates passing through the upstream particulatefilters 100 are trapped by the downstream particulate filter 300. Thiscombination significantly reduces the total amount of thecarbon-containing particulates released to the air.

[0171] After the Diesel engine 10 is driven for some time, the throttlevalve 60 is partly closed under control of the engine control ECU 30 todecrease the concentration of oxygen in the exhaust gas. Under thenormal driving conditions, the amount of intake air is greater than theamount of fuel injected into the combustion chamber. There isaccordingly excess oxygen. When the throttle valve 60 is partly closedto decrease the amount of intake air, practically all the intake oxygenis used up. Closing the throttle valve 60 to an excess level, however,results in an insufficient amount of the intake air to drastically lowerthe output or to cause failure of combustion of the injected fuel. Theengine control ECU 30 thus regulates the opening of the throttle valve60 to take in an appropriate quantity of the air including a necessaryand sufficient amount of oxygen, based on information regarding theengine speed and the accelerator opening. For simplicity of explanation,it is here assumed that the throttle valve 60 is closed at regularintervals. In the actual state, however, the throttle valve 60 iscontrolled at adequate timings by taking into account the accumulationof the exhaust gas emitted from the engine and the engine drivingconditions.

[0172] With a decrease in concentration of oxygen in the exhaust gas,the nitrate ion stored in the active oxygen release agent 302 on thedownstream particulate filter 300 is decomposed on Pt and producesactive oxygen, which reacts with the particulates trapped on the filter.This causes the carbon-containing particulates trapped on the filter tobe converted into carbon dioxide and water, while the nitrate ion losesoxygen on Pt and is thus converted to harmless nitrogen. These harmlesssubstances are then released to the exhaust gas.

[0173] Under the normal driving conditions where excess oxygen ispresent in the exhaust gas, the particulate filter 300 traps thecarbon-containing particulates and takes in the nitrogen oxides in theexhaust gas together with oxygen in the form of nitrates. Under theconditions where oxygen in the exhaust gas is deficient, the particulatefilter 300 converts the trapped carbon-containing particulates and theintake nitrogen oxides into harmless substances like nitrogen and water.The particulate filter 300 traps a trace amount of carbon-containingparticulates passing through the upstream particulate filters 100, so asto lower the total quantity of the carbon-containing particulatesreleased to the air, while regulating and reducing the nitrogen oxidesthat have not been treated on the upstream particulate filter 100.

[0174] Even if a large amount of carbon-containing particulates aredischarged from the Diesel engine 10, most of the particulates aretrapped by the upstream particulate filters 100, whereas a trace amountof the particulates reach the downstream particulate filter 300. Even insuch cases, the downstream particulate filter 300 is free from carbonpoisoning and can efficiently reduce the carbon-containing particulatesand nitrogen oxides included in the exhaust gas.

[0175] C. Third Embodiment

[0176]FIG. 14 conceptually illustrates an emission control device of athird embodiment applied to the Diesel engine 10. As illustrated, a maindifference of the emission control device of the third embodiment fromthe emission control device of the second embodiment is that thesupercharger 20 is replaced by a surge tank 40 for relieving a variationin flow rate. In the emission control device of the third embodiment,the surge tank 40 is located downstream of the particulate filters 100having the spontaneous regenerating function, and the particulate filter300 with the active oxygen release agent carried thereon is locatedfurther downstream of the surge tank 40.

[0177] In the emission control device of the third embodiment having theabove construction, the exhaust gas emitted from the Diesel engine 10 isflown with a variation in flow rate into the upstream particulatefilters 100. Large carbon-containing particulates in the exhaust gascollide with and adhere to the metal fibers constituting the particulatefilters 100. The particulate filters 100 can thus effectively trap theseparticulates. The exhaust gas passing through the particulate filters100 is flown into the surge tank 40, where the variation in flow rate isrelieved. This is ascribed to the following phenomenon. In general, whena fluid flowing in a narrow pathway is discharged to a chamber of alarge volume, for example, a surge tank, the abrupt expansion of thearea of the pathway causes an abrupt decrease in flow rate. Thedecreased flow rate is converted into a pressure. Unless the surge tankhas an extremely small volume, the pressure in the tank is notdrastically varied as the variation in flow rate. The flow rate of thefluid out of the surge tank to a downstream pathway is thus notsignificantly varied as the flow rate of the fluid into the surge tank.Namely the conversion of the flow rate into the pressure effectivelyrelieves the variation in flow rate.

[0178] In the emission control device of the third embodiment, theexhaust gas passing through the particulate filters 100 has the relievedvariation in flow rate by means of the surge tank 40 and is flown intothe downstream particulate filter 300. As described previously, theparticulate filter 300 has the porous structure, which traps thecarbon-containing particulates while the flow of exhaust gas passesthrough the filter. The exhaust gas having the relieved variation inflow rate by means of the surge tank 40 is flown into the downstreamparticulate filter 300. The downstream particulate filter 300 can thusefficient trap the fine carbon-containing particulates as describedabove.

[0179] The exhaust gas is flown with the variation in flow rate into theupstream particulate filters 100. This enables trapping of finercarbon-containing particulates, because of the reason described above.There is accordingly very little possibility that relatively largecarbon-containing particulates are flown into the downstream particulatefilter 300 and clog the filter. By taking into account this advantage,the particulate filter 300 may be constructed to have finer pores andtrap finer carbon-containing particulates. Since the variation in flowrate is relieved, the finer carbon-containing particulates do not movearound on the surface of the particulate filter 300 due to the variationin flow rate but are trapped immediately. This effectively preventssecondary aggregation of the particulates to a greater size to clog theparticulate filter 300.

[0180] The downstream particulate filter 300 additionally traps the finecarbon-containing particulates, which are quickly treated with activeoxygen released from the active oxygen release agent 302 carried on thefilter. The quick treatment of the trapped particulates allows trappingof newly inflow particulates and thereby enhances the trappingefficiency of the particulates.

[0181] C-1. Modifications

[0182] The emission control device of the third embodiment may beactualized in a diversity of applications. FIG. 15 conceptually showsvarious modifications. In the structure of FIG. 14, the surge tank 40 isdisposed between the upstream particulate filters 100 and the downstreamparticulate filter 300 as the means of relieving the variation in flowrate. As shown in FIG. 15(a), the surge tank 40 may be replaced with aflow-restriction element like an orifice 42. The flow-restrictionelement interferes with transmission of the variation component of theflow rate to the downstream side of the flow-restriction element, thusrelieving the variation in flow rate of the exhaust gas flown into thedownstream particulate filter 300. The supercharger 20 may be provided,in place of the orifice 42. Like the orifice 42, the supercharger 20functions as the flow-restriction element and relieves the variation inflow rate of the exhaust gas flown into the downstream particulatefilter 300.

[0183] As shown in FIG. 15(b), the flow-restriction element like theorifice 42 may be located downstream of the particulate filter 300. Theexhaust gas has difficulty in passage through the flow-restrictionelement like the orifice 42, so that the variation in flow rate is notdirectly transmitted to the downstream side of the flow-restrictionelement. The variation component of the flow rate is thus converted intoa pressure variation in the upstream side of the flow-restrictionelement. Because of the restriction by the orifice 42, the increasingflow rate on the upstream side of the orifice 42 can not be directlytransmitted through the orifice 42 and thereby increases the pressure onthe upstream side of the orifice 42. The increased pressure is loweredagain with recovery of the flow rate to the previous level on theupstream side of the orifice 42. At least part of the variation in flowrate is converted into a pressure variation on the upstream side of theflow-restriction element like the orifice 42. This ensures the relievedvariation in flow rate on the upstream of the flow-restriction element.Namely the flow-restriction element like the orifice 42 provideddownstream of the particulate filter 300 as shown in FIG. 15(b)preferably relieves the variation in flow rate of the exhaust gas flowninto the particulate filter 300.

[0184] A small-sized control catalyst (for example, an oxidationcatalyst) 400 for controlling the emission may be provided, in place ofthe flow-restriction element like the orifice 42 as shown in FIG. 15(c).The control catalyst 400 has flow resistance and thus relieves thevariation in flow rate of the exhaust gas flown into the particulatefilter 300 according to a similar mechanism to the mechanism with theflow-restriction element like the orifice 42. The control catalyst 400provided as the flow-restriction element additionally reduces airpollutants passing through the particulate filter 300. As describedabove, the particulate filter 300 traps the carbon-containingparticulates in the exhaust gas and utilizes active oxygen forregulation and reduction of the trapped carbon-containing particulates.The particulate filter 300 may, however, not effectively trap gaseousair pollutants like SOF but may allow passage of such air pollutants.The control catalyst 400 provided as the flow-restriction elementdownstream of the particulate filter 300 reduces the air pollutantspassing through the particulate filter 300. The control catalyst 400 canalso reduce intermediate products (for example, carbon monoxide), whichare produced in the course of combustion of the carbon-containingparticulates and are leaked from the particulate filter 300 undercertain conditions.

[0185] In the modified example shown in FIG. 15, the flow-restrictionelement, such as the surge tank 40 or the orifice 42, is disposed eitherdownstream or upstream of the particulate filter 300. Theflow-restriction element may be some combination of theseflow-restriction elements. FIG. 16 shows one example of suchcombination. In this example, the supercharger 20 as theflow-restriction element is disposed upstream of the particulate filter300, and the small-sized control catalyst 400 as the flow-restrictionelement is disposed downstream of the particulate filter 300. The Dieselengine is generally provided with a supercharger for higher output. Thesupercharger 20 is thus used as the flow-restriction element on theupstream side of the particulate filter 300, while the control catalyst400 is used as the flow-restriction element on the downstream side ofthe particulate filter 300. The upstream flow-restriction element alsofunctions as the supercharger, and the downstream flow-restrictionelement functions as the control catalyst. This arrangement attains thetotally effective configuration.

[0186] The above embodiments and their modifications are to beconsidered in all aspects as illustrative and not restrictive. There maybe many modifications, changes, and alterations without departing fromthe scope or spirit of the main characteristics of the presentinvention. All changes within the meaning and range of equivalency ofthe claims are therefore intended to be embraced therein.

[0187] For example, in the above embodiment, the particulate filter mayhave the metal non-woven fabric. Another known filter like a ceramicfilter may be applicable for the particulate filter.

[0188] In the embodiments discussed above, the Diesel engine 10 isprovided with the supercharger 20. The arrangements of the respectiveembodiments are, however, also applicable to the Diesel engine withoutthe supercharger.

INDUSTRIAL APPLICABILITY

[0189] As described above, in the emission control device of the presentinvention, the upstream particulate filter and the downstreamparticulate filter function complimentarily to ensure effective andefficient regulation and reduction of the carbon-containing particulatesin the exhaust gas. The technique of the present invention is thuspreferably applied to the emission control devices that reduce theemission from various internal combustion engines, as well as theemission control devices for a diversity of vehicles and ships withinternal combustion engines as the power source. The technique of theinvention is also applicable to stationary internal combustion engines.

1. An emission control device that reduces carbon-containingparticulates included in a flow of exhaust gas from an internalcombustion engine, said emission control device comprising: a firstheat-resistant filter medium that traps hydrocarbon compounds and thecarbon-containing particulates included in the flow of exhaust gas in adispersive manner to bring the respective particulates and hydrocarboncompounds in contact with oxygen included in the exhaust gas, andthereby makes the trapped hydrocarbon compounds and the trappedcarbon-containing particulates subjected to combustion with the exhaustgas having a filter inflow temperature lower than a combustibletemperature of the carbon-containing particulates; and a secondheat-resistant filter medium that traps the remaining carbon-containingparticulates, which have not been trapped by said first heat-resistantfilter medium but have passed through said first heat-resistant filtermedium.
 2. An emission control device in accordance with claim 1,wherein said second heat-resistant filter medium is capable of trappingthe remaining carbon-containing particulates, which are smaller in sizethan the carbon-containing particulates collectable by said firstheat-resistant filter medium.
 3. An emission control device inaccordance with claim 1, wherein said internal combustion enginecomprises: a plurality of combustion chambers; an exhaust manifold thatunites flows of exhaust gas from said plurality of combustion chambersto at least one joint flow; and an exhaust pipe that leads the jointflow of exhaust gas united by said exhaust manifold to the air, saidfirst heat-resistant filter medium is disposed in said exhaust manifold,and said second heat-resistant filter medium is disposed in said exhaustpipe.
 4. An emission control device in accordance with claim 3, whereinsaid first heat-resistant filter medium is disposed at a specificposition where the flows of exhaust gas from said plurality ofcombustion chambers are united to the at least one joint flow, in saidexhaust manifold.
 5. An emission control device in accordance with claim1, wherein said first heat-resistant filter medium does not trap most ofmetal sulfate particulates but allows passage of the metal sulfateparticulates therethrough, the metal sulfate particulates being producedfrom metal components added to lubricating oil of said internalcombustion engine and sulfur in a fuel of said internal combustionengine and being suspended in the flow of exhaust gas.
 6. An emissioncontrol device in accordance with claim 1, said emission control devicefurther comprising a vane that is located on a pathway of the flow ofexhaust gas from said internal combustion engine, is driven by the flowof exhaust gas, and breaks down the particulates included in the flow ofexhaust gas, wherein said first heat-resistant filter medium is disposedupstream of said vane, and said second heat-resistant filter medium isdisposed downstream of said vane.
 7. An emission control device inaccordance with claim 6, wherein said internal combustion engine isprovided with a supercharger that utilizes fluidization energy of theexhaust gas to supercharge intake air of said internal combustionengine, and said vane is a turbine of said supercharger actuated by theflow of exhaust gas.
 8. An emission control device in accordance withclaim 1, said emission control device further comprising a controlcatalyst that is disposed in back wash of said second heat-resistantfilter medium to reduce air pollutants flown with the exhaust gas out ofsaid emission control device.
 9. An emission control device inaccordance with claim 1, wherein said second heat-resistant filtermedium has an active oxygen release agent carried thereon, the activeoxygen release agent taking in and holding oxygen in the presence ofexcess oxygen in its atmosphere and releasing the oxygen held therein asactive oxygen with a decrease in concentration of oxygen in theatmosphere.
 10. An emission control device in accordance with claim 9,wherein said second heat-resistant filter medium has a noble metalcatalyst belonging to a platinum group carried thereon, in addition tothe active oxygen release agent.
 11. An emission control device thatreduces carbon-containing particulates, which are included in a flow ofexhaust gas with a variation in flow rate emitted from an internalcombustion engine, using a filter material having a large number ofpores tangled in a three-dimensional manner, said emission controldevice comprising: a first heat-resistant filter medium that is composedof the filter material, makes the exhaust gas flown into the pores,which are greater in size than the carbon-containing particulates, andcauses the carbon-containing particulates to collide with and adhere toregions defining the pores of the filter material, thereby trapping thecarbon-containing particulates; a second heat-resistant filter mediumthat filters the flow of exhaust gas passing through said firstheat-resistant filter medium to trap the remaining carbon-containingparticulates included in the flow of exhaust gas; and a flow ratevariation mitigation module that mitigates the variation in flow rate ofthe exhaust gas flown into said second heat-resistant filter medium. 12.An emission control device in accordance with claim 11, wherein saidfirst heat-resistant filter medium traps hydrocarbon compounds and thecarbon-containing particulates included in the flow of exhaust gas in adispersive manner to bring the respective particulates and hydrocarboncompounds in contact with oxygen included in the exhaust gas, andthereby makes the trapped hydrocarbon compounds and the trappedcarbon-containing particulates subjected to combustion with the exhaustgas having a filter inflow temperature lower than a combustibletemperature of the carbon-containing particulates, and said secondheat-resistant filter medium has an active oxygen release agent carriedthereon, the active oxygen release agent taking in and holding oxygen inthe presence of excess oxygen in its atmosphere and releasing the oxygenheld therein as active oxygen with a decrease in concentration of oxygenin the atmosphere.
 13. An emission control device in accordance withclaim 12, wherein said flow rate variation mitigation module comprises asupercharger that is actuated by fluidization energy of the exhaust gasand supercharges intake air of said internal combustion engine.
 14. Anemission control device in accordance with claim 13, wherein said flowrate variation mitigation module further comprises a flow-restrictionelement that is disposed in back wash of said second heat-resistantfilter medium to restrict the flow of the exhaust gas.
 15. An emissioncontrol device in accordance with claim 14, wherein saidflow-restriction element is a control catalyst that reduces airpollutants included in the flow of exhaust gas passing through saidsecond heat-resistant filter medium.
 16. An emission control method thatreduces carbon-containing particulates included in a flow of exhaust gasfrom an internal combustion engine, said emission control methodcomprising the steps of: using a first heat-resistant filter medium totrap hydrocarbon compounds and the carbon-containing particulatesincluded in the flow of exhaust gas in a dispersive manner to bring therespective particulates and hydrocarbon compounds in contact with oxygenincluded in the exhaust gas; making the trapped hydrocarbon compoundsand the trapped carbon-containing particulates subjected to combustionwith the exhaust gas having an inflow temperature into said firstheat-resistant filter medium lower than a combustible temperature of thecarbon-containing particulates; and using a second heat-resistant filtermedium to trap the remaining carbon-containing particulates, which havenot been trapped by said first heat-resistant filter medium but havepassed through said first heat-resistant filter medium.
 17. An emissioncontrol method that reduces carbon-containing particulates, which areincluded in a flow of exhaust gas with a variation in flow rate emittedfrom an internal combustion engine, using a filter material having alarge number of pores tangled in a three-dimensional manner, saidemission control method comprising the steps of: making the exhaust gasflown into the pores, which are greater in size than thecarbon-containing particulates, and causing the carbon-containingparticulates to collide with and adhere to regions defining the pores ofthe filter material, thereby trapping the carbon-containingparticulates; mitigating the variation in flow rate of the exhaust gas;and filtering the flow of exhaust gas with the mitigated variation inflow rate, thereby trapping the remaining carbon-containing particulatesincluded in the flow of exhaust gas.